Nowadays, more than one Global Navigation Satellite System (GNSS) is available, which includes GPS, Galileo and GLONASS. A receiver supporting multi-specification LBS (location based service), wireless multimedia communication and broadcasting signals is becoming an expectation. Take multi-specification LBS as an example, such a receiver able to support multi-mode receiving for GNSS signals can enhance locating precision and access to more services. Among the GNSS systems, different signal frequency bands support different services. As more and more bands need to be supported, band overlapping occurs.
FIG. 1 generally shows frequency band distribution of GPS and Galileo systems. GPS is the U.S. navigation satellite system, which is a network of satellites continuously transmits high-frequency radio signals. The signals carry time and distance information that is receivable by a GPS receiver, so that a user can pinpoint the position thereof on the earth. Galileo, the emerging European satellite navigation system, offers higher signal power and more robust modulation that will enable users to receive weak signals even in difficult environments. When combined, Galileo and GPS will offer twice the number of satellite sources as currently available. This provides redundancy as well as greater availability for the user. As shown, the combination of GPS and Galileo basically has four bands, excluding SAR (Safe and Rescue) service. GPS and Galileo systems share some signal bands. For example, GPS L1 and Galileo E2-L1-E1 share the same band by using a specific modulation scheme, such as Binary Offset Carrier (BOC) modulation, to avoid interference.
A receiver receives the GNSS signals of various bands and down converts the signals to baseband signals. An RF front end of the receiver usually utilizes the down conversion method or direct digitization method to down convert the RF signals. In a conventional down conversion method, one local oscillator, one mixer and ADC (analog-to-digital converter) are needed for signals of one band. The cost is significantly high. To solve this problem, the direct digitization method is adopted. In the direct digitization method, a sampling frequency is selected to down convert the RF signals of various bands. However, the selection of the sampling frequency is sometimes difficult to accomplish. In addition, as mentioned, a special modulation such as BOC is applied to avoid interference and GNSS signals are spread with different PRN codes, however, a problem of SNR (Signal-to-Noise Ratio) degradation will still occur due to cross correlation noise if the bands of the down converted signals are overlapped.